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 <h1>HIERARCHY, SYMBOLS AND COMPONENT INSTANCES</h1>

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   <li>Describe complex circuits by using symbols.</li>
   <li>A component placed in a schematic is an instance of a 
      <a href="http://repo.hu/projects/xschem/xschem_man/tutorial_create_symbol.html">symbol</a>.</li>
   <li>A symbol hides the underlying circuit, showing only the interface (pins) to the parent schematic.</li>
   <li>Repetitive blocks (registers, amplifiers, comparators, logic gates, ...) can be used multiple times in a design.</li>
   <li>When creating a netlist the circuit is traversed all the way from the top level down to the lowest level leaves.</li>
   <li>At the lowest level we have primitive symbols, these have attributes telling the netlister how to describe them.</li>
   <li>A symbol definition is stored in a .sym file (inv.sym), while the circuit implementation is in a .sch file (inv.sch).</li>
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